1. Field of the Invention
The present invention relates to a heat-curable silicone rubber composition capable of curing through hydrosilylation reaction which is used for potting materials, coating materials, adhesives, liquid injection molding materials and so forth.
2. Description of the Prior Art
Heat-curable type silicone elastomer compositions which cure through hydrosilylation reaction have such advantages as curing rapidly and giving no reaction by-products, and are used for potting materials, coating materials, adhesives, liquid injection molding materials and the like.
However, because of difficulties in controlling the catalyst activity used for the hydrosilylation reaction, the heat-curable silicone elastomer compositions have been used in the form of two-part type compositions. In using such a two-part type composition for coating materials and the like, the two parts must be mixed together, which leads to workability problems such as complicated operations.
For control of catalyst activity at room temperature, therefore, a proposal has been made of a one-part type heat-curable silicone elastomer composition which incorporates a hydrosilylation reaction retarding agent. As the controlling agent, have been used, for example, nitrile compounds; carboxylates; stannous, mercuric and the like metallic compounds; sulfur compounds; benzotriazole; acetylene compounds; hydroperoxides and so forth. One-part type compositions containing such a controlling agent show good stability at room temperature. However, the addition of the reaction retarding agent to a composition in order to obtain long-term storage stability results in marked lowering in the curability of the composition. In order to retain curability of a composition, on the other hand, the addition amount of the controlling agent should be reduced, so that the resulting composition is poor in storage stability.
In view of the above, one-pack type heat-curable silicone elastomer compositions have been developed which are obtained by adding a platinum catalyst encapsulated in a silicone resin having a certain softening point (hereinafter referred to as "catalyst-containing resin") to an elastomer composition (Refer to Japanese Patent Publication (KOKOKU) No. 53-41707 (1978) and U.S. Pat. No. 4,481,341 corresponding to Japanese Pre-examination Patent Publication (KOKAI) No. 58-37053 (1983)). This method, however, has the problem that at the time of encapsulating the platinum catalyst in the silicone resin, a tiny amount of the catalyst deposits on the surface of the resin. The catalyst thus deposited on the surface of the catalyst-containing resin promotes curing of the silicone composition during storage. The one-part type elastomer composition obtained in this manner, therefore, are low in storage stability (the term "storage stability" hereinbelow refers to the property of retaining room-temperature stability and high-temperature curability for a long time).
To overcome the above problem, a composition in which the abovementioned catalyst-containing resin and hydrosilylation reaction retarding agent are incorporated separately has been proposed (Refer to Japanese Pre-examination Patent Publication (KOKAI) No. 4-46962 (1992)). The one-pack type heat-curable silicone elastomer composition thus obtained has good stability at room temperature over a long period. To obtain the stability, however, the reaction retarding agent must be added to the composition in an excess amount. The excess amount of the controlling agent permeates the catalyst-containing resin with the lapse of time, so that the compositions of this type are low in storage stability and their curability at high temperature is lowered with the lapse of time, thereby making them unsuited to practical use.